1. Field of the Invention
The present invention relates to an image forming apparatus, an image forming method, and a method of producing a printed material.
2. Description of the Related Art
Recently, digital printing machines applying an electrophotographic process are increasingly used in a production printing industry. Therefore, higher image quality and higher credibility are demanded of the digital printing machines applying the electrophotographic process. Particularly, improvement of uniformity in concentration of a halftone is strongly demanded of the digital printing machines.
For the purpose of achieving high image quality, the digital printing machines applying the electrophotographic process are equipped with an image processing unit that corrects image data by image processing. This image processing unit performs the image processing with multiple-bit data at high resolution of, for example, 1200 dots per inch (dpi) or 2400 dpi.
Moreover, the digital printing machine applying the electrophotographic process includes a photosensitive drum a surface of which functions as a photosensitive surface to be scanned, a light source that emits a laser beam, a polygon mirror that deflects the laser beam from the light source, a scanning optical system that guides the laser beam deflected by the polygon mirror to the surface of the photosensitive drum (surface to be scanned), and the like. The digital printing machine applying the electrophotographic process modulates a beam emitted from the light source based on image data, and irradiates the beam from the light source on the surface to be scanned, to scan the surface to be scanned with the beam, thereby forming an electrostatic latent image according to the image data on the photosensitive drum.
The digital printing machine applying the electrophotographic process having such a configuration uses a device that has multiple light emitting points such as a laser diode array (LDA) and a vertical cavity surface emitting laser (VCSEL). Thus, the digital printing machine applying the electrophotographic process can form an electrostatic latent image with resolution of, for example, 2400 dpi or 4800 dpi that is higher than that of the image data at 1200 dpi.
Furthermore, in the digital printing machine, parallel line dithering in which a region having uniform concentration (that is, a region of pixels having the same pixel value) in image data is replaced with a parallel line pattern is performed in some cases as a pseudo-halftone processing. The parallel line pattern expresses concentration in surface by changing each line width according to an original pixel value.
For example, in a technique described in Japanese Laid-open Patent Publication No. 2004-282344, resolution of a binary parallel line pattern generated by performing a pseudo-halftone processing in an image processing unit is increased and then interpolation and smoothing are performed, thereby suppressing roughness without changing the number of lines. In a technique disclosed in Japanese Patent No. 4026271, pixels are divided by small regions and resolution thereof is increased, and smoothing is performed on an edge portion of an image, to obtain high image quality.
When an electrostatic latent image of a parallel line pattern is formed on a photosensitive drum, if a distances between lines is short, a shape, an electric-field intensity distribution, and the like of the electrostatic latent image corresponding to one line are influenced by an adjacent electrostatic latent image. Particularly, if line intervals are short, a low electric-field intensity area in which adhesion of toner is unstable is generated. Therefore, in a digital printing machine, when the parallel line dithering is performed, it is preferable that the electric field intensity of a latent image of each line be equal to or higher than a threshold to keep a certain intensity at any position on the line, to make the concentration of an output image uniform.
However, in a parallel line pattern of low-resolution image data, an edge of each line has a rough stepped-shape. Because of this, in a parallel line pattern of low-resolution image data, distances between lines vary depending on positions of the lines, and influence of a latent image corresponding to an adjacent line varies, and therefore, an area in which the electric field intensity is partially low is generated. As a result, toner can adhere non-uniformly, or toner scattering can occur between lines. Accordingly, for the digital printing machine, it has been difficult to obtain uniform concentration when the parallel line dithering is performed on low-resolution image data.
To solve this problem, it is necessary to perform the parallel line dithering on higher-resolution image data in the digital printing machine. However, if the parallel line dithering is performed on higher-resolution image data, the amount of data to be transferred from the pseudo-halftone processing unit to a light-source driving circuit positioned downstream increases, and thus limiting productivity.
In view of the above, there is a need to provide an image forming apparatus, an image forming method, and a method of producing a printed material that can make concentration of a parallel line pattern uniform.